Scanning electron microscopes are highly versatile electron beam instruments that can provide images over 100 times more magnified than commercial optical microscopes. They have applications in a wide range of different research areas, including, medicine, biology, material science and microelectronics.
In principle, many analytical techniques can be combined with the normal operation of a SEM, but in practice, they are limited to a few routinely used add-on attachments. The main reason for this lies in the way SEMs are currently designed. At present, conventional SEMs include an electron gun, electromagnetic lenses, scan coils and apertures. The electromagnetic lenses are usually divided into two categories, condenser lenses that are placed immediately after the electron gun, and the objective lens located just before the specimen. Typical SEMs produce a high energy (1-30 keV) beam of electrons that is successively focused into a sub-micron probe and raster scanned over the sample's surface. This beam of electrons is normally referred to as the primary beam.
Unfortunately, most of the electrons and photons that are scattered back from the sample travel back towards the column and are therefore difficult to detect and analyze. Moreover, the distance between the lower pole-piece of the objective lens and the surface of the sample, commonly called the working distance, is relatively small, typically restricted to be between 3 to 25 mm. The acquisition of high resolution images requires that this distance be minimized: the working distance in many cases needs to be less than 5 mm. There is therefore little room in this design to mount detectors and spectrometers that can efficiently collect the secondary electrons and photons generated by the primary beam-specimen interaction or analyze their energy spectra with high resolution.
The SEM uses secondary electrons to form its most common form of topographic image. Secondary electrons are scattered electrons having energies of only a few eV. The secondary electron detector is placed to one side of the specimen so that it does not influence or obstruct the primary beam. This off-axis position, however, results in poor collection efficiency for the secondary electrons, typically well below 50% in most conventional SEMs.
Another popular mode of imaging in SEMs is to use more elastically scattered electrons known as back scattered electrons (BSEs). BSEs are defined to have energies ranging from 50 eV up to the primary beam energy. In conventional SEMs, BSEs have straight line trajectory paths radiating out in different directions, and a special purpose BSE detector is usually mounted just below the objective lens pole-piece to collect the wide-angle BSEs. The BSE detector usually takes the form of a side-entry attachment having a hole to allow passage of the primary beam. Due to its restricted angle of collection, the transport efficiency of BSEs reaching the detector is generally low, well below 50%. Some BSEs are inevitably detected by the secondary electron detector, creating a background current to the secondary electrons signal and degrading secondary electron contrast information. In general, the mixing of secondary electrons and BSEs is undesirable since BSEs provide poorer spatial resolution than secondary electrons. BSEs however carry stronger material contrast information and, for this reason, their separate detection is preferred.
Other commonly used SEM attachments allow for the detection of X-rays, infrared, UV and visible light radiation. An energy-dispersive X-Ray spectrometer (EDS), or a wavelength dispersive X-Ray spectrometer (WDS), enables the SEM to identify the presence of different metals. The Cathodoluminescence technique (CL) works by detecting infrared, UV or visible light radiation from certain specimens like semi-conductors and some organic materials. The spectrum and transient response of the CL signal can provide useful analytical information about the specimen. Although EDS, WDS or CL attachments are very useful, conventional SEM designs are not optimized for their use, precisely for the same reasons that preclude the high efficiency collection of secondary electrons and BSEs. In order not to obstruct the primary beam, their detectors must be placed to one side of the specimen. Moreover, since the working distance must be kept relatively small, only a small fraction of the total emission angle can be captured.
There are several instances where energy filtering of the scattered electrons provides important analytical information about the sample, however, the difficulty of employing high resolution spectrometers into conventional SEMs is well documented in L. Reimer, Scanning Electron Microscopy, Physics of Image Formation and Microanalysis, 2nd edition, chapter 5: Electron Detectors and Spectrometers, pp. 171-204, 1998. The technique of quantitative voltage contrast for instance, functions by monitoring energy shifts in the secondary electron spectra.
Although a variety of different electron/energy detectors have been proposed, no SEM capable of using a variety of detectors has emerged. Instead, special purpose Electron Beam Test (EBT) columns have been built, as for example detailed in John T. L. Thong, Electron Beam Testing Technology, New York, Plenum, 1993.
Accordingly, there remains a need for a flexible SEM, capable of detecting various emissions or scatterings from a specimen.